A solar cell is an electrical device that converts the energy of light directly into electricity by the photovoltaic effect. It is a form of photoelectric cell (in that its electrical characteristics—e.g. current, voltage, or resistance—vary when light is incident upon it) which, when exposed to light, can generate and support an electric current without being attached to any external voltage source.
The efficiency of a solar cell may be broken down into reflectance efficiency, thermodynamic efficiency, charge carrier separation efficiency and conductive efficiency. The overall efficiency is the product of each of these individual efficiencies.
Bulk-heterojunction polymer solar cells have emerged as an attractive type of cost-effective solar energy electrical power transforming device. Recently, great progress in the development of new photo-harvesting materials and device optimizations have been achieved in this field, resulting in the significant improvement of the power conversion efficiencies of polymer solar cells from around 1% to higher than 8.0%. The rational design and fine tailoring of the molecular structures of donor polymers significantly contributed to these prominent advances. Among all kinds of donor polymers, push-pull conjugated polymers, which consist of alternating electron-rich and electron-deficient units have been most extensively developed and have dominated the library of donor materials for polymer solar cells, because their intrinsic optical and electronic properties can be readily tuned to the desired situation by controlling the intramolecular charge transfer from donor unit to acceptor unit.
Ideal conjugated polymers as donors for solar cells should have a narrow band gap for a broad light absorption, a deep highest occupied molecular orbital for a high open-circuit voltage, and a high charge mobility for a low series resistance. A push pull strategy to achieve these types of conjugated polymers is to combine a weak electron-rich moiety with a strong electron-deficient moiety.